Method of making heat transfer tubes



July 2, 1935; H K EQ 2,006,738

METHOD OF MAKING HEAT TRANSFER TUBES Filed June 26, 1935 Patented July 2, 1935 UNITED STATES PATENT OFFICE Stewart H. Kilmer,

Ambridge, Pa., assignor to Spang, Chalfant 85 00., Inc., Pittsburgh, Pa., a corporation of Pennsylvania Application Jim 26, 1933, Serial No. 677,635 1 Claim. (01. 29-1513) My invention relates generally to the manufacture of heat-transfer tubes which comprise a tubular core upon which are shrunk a plurality of cast metal fin rings, and its principal object 5 is the provision of 'a new and improved heattransfer tube which will be more desirable and will possess improved heat-transference properties.

In the present practice the cores of such heattransfer tubes are either cold drawn or hot rolled seamless tubing. The use of cold drawn tubing has this marked advantage. ts perimetral surface is of substantially uniform diameter, and fin rings of uniform internal diameter may be successfully shrunk thereon. However,

the grain structure of such tubing is not refined and therefore its life in service is. relatively short, owing to the extreme temperatures to which it is exposed. Experience has shown that hot-rolled tubes, either as rolled or normalized, are, by reason of their superior grain structure, better than cold-drawn tubes for the class of service to which this invention relates. Probably the smoothness of the surface strains of hotrolled tubes is greater and the stressed condition of the surface of the cold-drawn tube is more susceptible to corrosion thanthe surface of the hot-rolled tubes.

As the tubes are subjected to changes of heat, it is desirable that the tube and the rings be as nearly as possible of the same composition and structure. This will reduce the thermo-couple effects as these effects are most pronounced with couples of unlike metals. It is of the utmost importance that the fit between the tube and the rings shall be uniform and intimate. This can be accomplished only by holding the diameters of the tubes and of the openings in the rings within a. very. close toleration limitation.

To refine the grain structure, cold-drawn tubing has to be heat-treated. but with the objectionable results that scale formation occurs and the uniformity of its perimetral surface is 45 destroyed so that the fin rings no longer shrink into close, uniform contact with the core.

In some instances, cold-drawn tubes, after heat-treatment, have been again rolled to remove the scale; but the result is a non-round 5o tube. I; i The result of loose or ill-fitting fin rings is an impairmentfiof the heat-transference between the core and the fin rings as pockets are formed between the core and the rings and heat-trans- 55 ference is by convection.

Hot-rolled seamless tubes are inmore general use as cores for heat-transfer tubes because of their superior grain structure as compared with cold-drawn tubes, but the external diameter of a hot-rolled tube varies materially throughout its 5 length and it has a taper from one end to the other. The surface is rough and scaled.

Therefore, with a hot-rolled core, fin rings of different internal diameters are required to properly fit different portions of the tube. This adds 10 to the labor and expense of manufacture. If the internal diameter of the ring he too small, the ring may break in cooling, and if the broken ring be an intermediate one, a new ring must be cast in its place: On theother hand, if the ring 15 has too large an internal diameter, it may be actually loose or air pockets may be formed between the core and the ring, in either case impairing the efliciency of the heat-transference.

A loose or ill-fitting ring may cause .the tube to 20 burn out at that point.

My improved heat-transfer tube has a core of substantially constant external diameter and a smooth perimetral surface free from scale. As a result, the fin rings used must be of uniform inter- 25 nal diameter properly sized to be shrunk onto the core with a close and uniform fit.

Therefore, no air pockets are formed in my tubes and no heat-transference by' convection takes place. Again, the rings neither become loose nor do they break upon cooling. The heattransference is expedited and uniform throughout the entire series of rings and throughout the entire area of the individual rings.

In my improved heat-transfertube the core is formed of a length of hot-rolled seamless tubing which is finished, either by grinding or machining to produce a smooth perimetral surface of substantially uniform diameter from end to end, free from scale.

I employ the proper number of in: rings having the proper uniform internal diameter so that when expanded by heating, mounted on the tube and shrunk in place, they will fit the core snugly and uniformly, thus producing a durable struc- 5 ture havingthe highest heat-transference .properties. V

In the accompanying drawing, which is a side elevation partially in section of a heat-transfer tube which embodies the principles of my invention. I represents the core which is a hot-rolled tube whose perimetral surface has been reduced by machining or grinding or both to produce a substantially uniform exterior diameter throughout the length-of the tube and a smooth, uniform surface 2, free of scale and pits.

The fin rings comprise a collar I and a plurality of radially extended fins l.

The core is first prepared by machining and grinding the surface of the lengthof hot rolled tubing, the tubing being intentionally rolled oversize to allow the proper finishing operation.

This process permits a more accurate computation-of the tolerance necessary for successfully shrinking the cast ring on the core.

The cast metal fin rings are machined to a uniform internal diameter so that they may be used indiscriminately in assembling them on the \tube, the proper shrink interference being calculated and provided for in the machined diameters.

The rings are then' heated, slipped into place on the core, and then shrunk in place.

The heat-transfer tubes which are the product of my improved process are superior to the present product where the fin rings are shrunk onto a hot-rolled tube without machining or grinding the tube surface, the reason for this being that the machined or ground surface on which the fin rings are shrunk is in contact with the whole area of the core of the rings which results in a uniformity of unit pressure between the two bodiesandasthefin ringsareshrunkonthe tubing there is no chance for the scale to form between the tube and the ring as the growth of the scale comes from the tube and not the cast -fin ring. The seal between the fin rings and the tubing in my heat-transfer tube made by my from the fin ring to the'tube or vice versa at a very high efllciency of heat conductivity. Y Iclaim:--

Anv improved process of commercially making a finned-ring heat transfer tube which comprises forming a hot rolled tube to an oversized dimen sion, machining said tube to a uniform diameter rings into place on said tube, and permitting them to shrink thereon in cooling.

process furnishes a path for conducting heat 

